Resin floor - And the cavity underneath

Hello everyone. We have had an epoxy resin floor in the garage for about 1 year. Unfortunately, it has turned out that a cavity has formed between the concrete slab and the epoxy resin floor over an area of about 1/2 m2. The epoxy resin floor apparently does not adhere to the concrete ceiling at this spot. When you tap on this area, it sounds hollow. What solutions are there to fix this problem? Thanks for the answers. Best regards

The epoxy probably did not adhere to the concrete due to moisture. The water must come out. Sand with a floor sander, let dry, then apply ABodentiefgrund, which binds residual water. Then epoxy again.

Water cannot be the problem because the foundation slab has already existed for some time or the garage was covered when the resin floor was installed. Thus, the substrate was dry as dust. I believe the primer therefore did not adhere properly, hence the cavity. What will it approximately cost if I hire another company? Is the only option to grind down and apply new primer and floor? Would the possibility be to open a small spot and inject a substance and then try to spread it as well as possible and then press the spot firmly down? Like an adhesive?

Injecting usually doesn't work. Possibly liability issues because oil/grease was on the concrete? Opening up and renewing is the safest option.

Are there perhaps other ideas besides a complete renovation?

I don't see the problem with [auf und neu]. That stuff doesn't stick to the floor, right? So use a chisel, lift it up and break it out. Sand the edges where it sticks well, dust it off and spot repair.